138 TESTS OF ELECTRIC EXCITATION. 



CHAPTER XVI. 



MECHANICAL ACTION CONSIDERED AS A TEST OF ELEC- 

 TRIC EXCITATION. POPULAR THEORIES OF ELEC- 

 TRICITY. 



A S of elementary molecules, so it may be said 

 of the electric ether: all we know of it is its 

 power of transmitting action. For this special 

 reason, light and freely movable bodies such as 

 feathers, straws, pith-balls, and flexible gold-leaf 

 are resorted to for receiving and developing me- 

 chanical impulses transmitted by the electric ether. 

 For convenient use two balls are commonly em- 

 ployed, made of the pith of elder, and suspended 

 by flexible threads attached to an insulating glass- 

 handle, as represented in Fig. 52. This simple 

 apparatus is called an " Electroscope," from two 

 Greek words signifying " Electricity I behold ; " 

 the movements of these balls being considered 

 equivalent to an exhibition of the transference of 

 impulses by the electric ether. 



On bringing the pith-balls near a piece of amber, 

 sealing-wax, or other resinous 

 substance, after putting in mo- 

 tion the electric ether pervad" 

 ing it by friction, the balls 

 are seen to move directly 



Pig. 52- 



